The present invention is related to charger circuits for battery cells. More particularly, the present invention is related to a protection circuit that prevents excessive charging of a battery cell when a fault is present in the charger system.
Rechargeable lithium bases batteries, including Lithium-Ion batteries and Lithium-Polymer batteries, are used in portable applications such as cellular telephones. Lithium batteries are sensitive to excessive voltages. Without a suitable safety circuit overcharging may compromise the batteries reliability. A shunt regulator is often employed to regulate the charging voltage to the lithium battery.
An example shunt regulator charging system for a lithium battery is shown in FIG. 1. As shown in FIG. 1, the shunt regulator charging system includes a power source (PS), a shunt regulator (102), and a lithium battery (BATT). The power source (PS) includes a voltage source (VS) and a source resistance (RS). The shunt regulator (102) includes a NMOS transistor (MN), a PMOS transistor (MP), an amplifier (AMP), and three resistors (R1-R3).
In operation the power source provides a charging current (I) to the lithium battery through source resistance RS, PMOS transistor MP, and resistor R3. Resistor R3 converts the charging current (I) into a voltage (VSNS), which is used by other circuitry (not shown) to control the activation of transistor MP. PMOS transistor MP is activated during normal charging operations. Resistors R1 and R2 form a voltage divider that provide a feedback signal to amplifier AMP. Amplifier AMP compares the feedback signal to a reference voltage (VREF) and provides a control signal to transistor MN. Transistor MN, amplifier AMP, and resistors R1-R2 together operate as a shunt regulator that regulates the input voltage (VIN). The shunt regulator provides safe charging of lithium battery BATT by limiting the charging voltage (input voltage) similar to a zener diode.
A shunt regulator system includes protection to prevent a battery from overcharging when the shunt regulator is active and a xe2x80x9cfaultxe2x80x9d condition is detected. The system may be implemented in an integrated circuit. A control circuit is used to actuate one or more switches when one or more xe2x80x9cfaultxe2x80x9d conditions are detected. The switches provide a conduction path to shunt current away from the battery such that the battery does not charge beyond a safe level. The control circuit may include a comparator that compares the battery voltage to a predetermined level, which may be adjusted based on various system requirements. The fault condition may result from defects and/or misuse, including: semiconductor processing defects such as a shorted resistor, manufacturing assembly defects such as improperly connected or defective components, utilizing an illegal charger, as well as others. The switches prevent the battery from being overcharged when fault conditions occur.